<!--实现城市列表右侧首字母手指滑动可以和列表联动-->
<template>
	<ul class="list">
		<!--可以在标签内绑定事件及方法-->
		<!--@touchstart.prevent="handleTouchStart"阻止事件冒泡-->
		<li class="item" 
			v-for="item of letters" 
			:key="item"
			:ref="item"
			@touchstart.prevent="handleTouchStart"
			@touchmove="handleTouchMove"
			@touchend="handleTouchEnd"
			@click="handleLetterClick"
		>
			{{item}}
		</li>
	</ul>
</template>

<script>
	export default{
		name:'CityAlphabet',
//		从父组件传递的参数需要在此声明,名称:类型
		props:{
			cities:Object
		},
//		这是一个计算属性
//类似于方法
		computed:{
			letters (){
				const letters = []
				for(let i in this.cities){
					letters.push(i)
				}
				return letters
			}
		},
		data (){
			return {
				touchStatus:false,
				startY: 0,
				timer:null
			}
			
		},
//		动态更新
		updated (){
			this.startY = this.$refs['A'][0].offsetTop
		},
		methods:{
			handleLetterClick(e){
				this.$emit('change',e.target.innerText);
			},
			handleTouchStart(){
				this.touchStatus = true
			},
			handleTouchMove(e){
				if(this.touchStatus){
					if(this.timer){
						clearTimeout(this.timer)
					}
					this.timer = setTimeout(() =>{
						const touchY = e.touches[0].clientY - 79;
						const index = Math.floor((touchY - this.startY) / 20);
						if(index >= 0 && index < this.letters.length){
							this.$emit('change',this.letters[index])
						}
					},16)
					
					
					
				}
			},
			handleTouchEnd(){
				this.touchStatus = false
			}
		}
	}
</script>

<style lang="stylus" scoped>
@import '~styles/varibles.styl'
	.list
		display:flex
		flex-direction:column
		justify-content:center
		position:absolute
		top:1.58rem
		right:0
		bottom:0
		width:.4rem
		.item
			line-height:.4rem
			text-align:center
			color:$bgColor
</style>